linux-kernel - Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

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Date:   Fri, 26 Jan 2018 18:53:13 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Hansen, Dave" <dave.hansen@...el.com>,
        Liran Alon <liran.alon@...cle.com>,
        Laura Abbott <labbott@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Borislav Petkov <bp@...e.de>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>,
        Radim Krcmár <rkrcmar@...hat.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Peter Anvin <hpa@...or.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        KVM list <kvm@...r.kernel.org>
Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation

On Fri, 2018-01-26 at 18:44 +0000, Van De Ven, Arjan wrote:
> your question was specific to RSB not BTB. But please show the empirical evidence for RSB ?

We were hypothesising, which should have been clear from:

On Fri, 2018-01-26 at 09:11 +0000, David Woodhouse wrote:
> Likewise if the RSB only stores the low 31 bits of the target, SMEP
> isn't much help there either.
> 
> Do we need to look again at the fact that we've disabled the RSB-
> stuffing for SMEP?

... and later... 

On Fri, 2018-01-26 at 17:31 +0000, David Woodhouse wrote:
> Note, we've switched from talking about BTB to RSB here, so this is a
> valid concern if the *RSB* only has the low bits of the target.

I'm glad to hear that it *isn't* a valid concern for the RSB and the
code in Linus' tree is correct.

Thank you for clearing that up.

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